This project uses 345 randomly selected hospitals and over 170,000 randomly selected patients from these hospitals to examine factors contributing to length-of-stay in U.S. hospitals. The project includes descriptive analyses, testing of existing and construction of a new patient mix index based on severity/comorbidity considerations, examination of clinical tracer conditions, testing of clinical hypotheses related to length-of-stay and constructions, of stochastic models to describe and predict patient-day level behavior in the hospital. The factors considered include both those which must be controlled (e.g., age sex severity/comorbidity) and those which are manageable or influenceable (utilization, timing of admissions, complications, payment type). Thus analyses results will be both directly policy-relevant and useful for subsequent policy analyses based on length-of-stay as an indication of resource use and risk of developing complications.